Electric gas cleaner



,` 1940 5 SheebS-Sheet l Filed Sept. 5

.NIN Il CHAPLES W WHL/MM5 c@ fNr/wrog Sept- 29, 1942. v C. w. WILLIAMS2,297,601

ELECTRIC GAS CLEANER i, m' Filed Sept: 3, 1940 3\Sheeks -Sheet 2 mi l ne2 LLI Y N CHARLESWW/LL/AMJMJm/wro 12,000 volts) between the' PatentedSept. 1942 ELECTRIC Gas CLEANER Charles W. Williams, Louisville, Ky.,assignor to American Air Filter Company, Inc.,

Louisville,

Ky., a corporation of Delaware Application September 3, 1940, Serial No.355,277

(ci. 18s-7) 8 Claims.

duct; and an interstitial dielectric separator substantially occupyingthe gas now space of the duct between electrodes. l

This invention relates to the art of cleaning gases electrically. In thepractical application of this art, gas borne "dust particles areelectrically charged by impressing high ionizing voltages betweenlaterally spaced electrodes to create an electrical field and passingthe gas longitudinally through the field -so created. Particles chargedor ionized in this manner` gravitate slowly across the gas flow towardoppositely charged surfaces upon which they are intended ultimately toprecipitate and collect.

To create an electrical field, ionizing voltages ranging from 30,000 to4more than 80,000 volts have been employed. Those high voltagesnecessitate wide spacing between electrodes, to avoid disruptive ashovers," while wide spacing, coupled with slow lateral movement ofcharged particles, tend to maximize the contact time required betweenthe gas and the collecting surfaces to effect cleaning. Under suchconditions the use of low gas velocities and long collecting surfaces ispromoted.

It has been proposed to reduce contact or cleaning time by rst passingthe gas through a. shallowelectric charging or ionizing field, createdby impressing high voltages between the adjacent widely spaced shortelectrodes of a laterally arranged series, and then passing the chargedgas through a deep electric collecting field, created by impressing arelatively low voltage (e. g. adjacent narrowly spaced long plate-likeelectrodes of a'laterally arranged series. AWhile such arrangementappears to eect a reductionV in cleaning time over.

other arrangements in which the charging and collecting neld are thesame, the optimum results appear to be limited.

With the collection of dust accomplished,y the v l problem of itskremoval is presented. `Up to the present time-this has been donelargely. .by jarring, scrapingl or similarly dislodging `the collecteddustfrom the collectingY surfaces or by iiushing such vsurfacesso as towash the dust therefrom.` As indicated in Patent No. 1,800,529, cleaningmethods of this general. character involve many diiiiculties which leadto many com- 65 plexities. In fact, they have been so highlyunsatisfactory that a considerable part of the time and effort spent bythose actively engaged in the general development oi' this art hasnecessarily been diverted to the problem of removing collected. dust.

An important object of this invention is to avoid or minimize theforegoing limitations and e objections, and, more particularly, toprovide an improved form oi' device for effectively cleaning gaselectrically, wherein the use of lower voltages and higher velocitiesare promoted and the removal of collected dust is facilitated.

'A further object is to provide in a device o f this character aninexpensive dust collecting medium which can be easily and quicklyremoved and replaced.

A still furtherobiect is to provide a compact device having desirableoperating characteristics, such as lowsoperating voltage, high gashandling capacity, low gas iiow resistance and high gas cleaningeiiiciency, and minimum maintenance requirements, such as low powerconsumption and low reconditioning expense.

. Another important object is to maintain an effective electrostaticcharge or4 potential upon a dielectric lter` mediumf through which gasis passed.

I have discovered that the foregoing objectives may be largely attainedby occupying the electric eld, created between suitably spacedinterstitial upstream and downstream electrodes, with an interstitialdielectric separator comprising a dielectric air filter medium, andpassing a gas successively through the interstices of the upstreamelectrode, the separator and the downstream electrode..

In other words,the gas is passed through the openings of an interstitialdielectric separator, while a potential difference is maintainedbetweenthe upstream and downstream faces of the separator. With anarrangement of this character, I have been able to obtain excellentcleaning results even when yusing operating voltages as low as 5,000volts and gas velocities as high as 300 F. P. M. The danger of iiashovers" is substantially nil since theV spacing of electrodes can bevaried sub-v stantially within safe limits'without appearing toaii'eci;A the 'cleaning eiiiciency v suspended particles, .collected ,inythis way have ylargelyl been deposited upon the air filter medium hencetheir removal maybe effected simply by removing the filter. Since theforegoing results are possible of attainment with to any appreciableextent. Fume, smoke and other the use ot an inexpensive `filter having alow gas now resistance, it will be readily appreciated that thearrangement makes possible the attainment of excellent operatingcharacteristics in an electric gas cleaning device having minimummaintenance requirements.

The invention is illustrated in the accompanying drawings wherein:

Figure 1 is a vertical section taken through one embodiment of theinvention in its operative gas cleaning position;

Figure 2 is an enlarged fragmentary view taken from Figure l, in whichthe details are exaggerated for the sake of clearness;

Figure 3 is a similarly enlarged fragmentary viewv taken at right anglesto Figure 2;

Figure 4 is a partly broken view of the upstream end of the device;

Figure 5 is a partly broken view of the downstream end of the device;

Figure 6 is a schematic view of an alternative arrangement for carryingout the invention;

Figure 7 is a practical embodiment of the arrangement shown in Figure 6;and

Figure 8 is a detail of the air filter structure shown in Figure '7.

The embodiment, used to illustrate the invention, is shown as mounted inoperative position within a gas duct D composed of wood. As shown, itincludes interstitial upstream and downstream electrodes I and 2, whichextend across the duct D and which are spaced from each other. 'Iheseelectrodes may be composed of any suitable material, such, for example,as ordinary metal window screening material having a mesh large enoughto avoid appreciable obstruction to the gas ilow. The screen electrodesI and 2 are mounted on shallow rectangular frames la and 2a and arrangedfor connection to a suitable source of direct current which is indicatedat 3. The upstream and downstream electrodesA are shown as beingconnected to the positive and negative sides respectively of the directcurrent supply 3 but this is not essential and such connections may bereversed. The experiments conducted with this device indicate that,while ,good cleaning results can be secured with an operating voltage of5,000 volts, better results may possibly be obtained with highervoltages. For this reason, a higher voltage of, say, 12,000 volts, issuggested but even that may be raised or lowered if desired.

When the desired voltage is impressed on the electrodes, an electricfield will be created between electrodes. Within this field thedielectric separator is placed. The dielectric separator includes aninterstitial filter-medium 4 which possesses dielectric properties.While various filter mediums may be used, better results appear possibleof attainment with those composed of short fibers. Particularlygoodresults have been obtained with a paper medium composed of a pluralityof superimposed separable layers of sheet-like fibrous material, whereineach sheet comprises a thin matted net work of short fibered wood pulp,characterized by a multitude of fine air strainer openings. A highlysatisfactory medium of this character is disclosed in Patent No.1,897,976 granted February 14, 1933.` The medium may be stretchedstraight across the gas flow with its upstream and downstream faces,either in contact with or minutely spaced from the upstream anddownstream electrodes respectively. This arrangement produces excellentcleaning results, but has the disadvantage of introducing a gasiiowresistance high enough to limit the gas flow velocities to valuessubstantially under feet per minute. For that reason the medium ispreferably arranged in zigzag form. to provide a face area large enoughto insure a satisfactorily low gas flow resistance with velocitiesranging as high as 300 feet per minute.

With the medium in zigzag form, a stiffener or cardboard spacer 5 ispreferably arranged in each fold of the medium to support it andlaterally corrugated to maintain free gas iiow spaces leading to andfrom each fold. The medium 4 and spacers 5 may be mounted within theconfines of an enclosing frame 6, having open upstream and downstreamgas flow faces, and held therein by retaining flanges 6a, which extendlaterally inward from the frame a short distance over the upstream anddownstream margins of the filtering medium, as a whole.

The resulting filtering unit should be arranged between electrodes withits opposed faces in substantial contact therewith. As indicated before,if actual physical contact is not made, the spacing between the mediumand the electrodes should be limited to minute distances in order tomaintain an effective electrostatic relation between opposed surfaces.While this arrangement is capable of producing good results, I havefound Vthat equally good and possibly better results can be secured withless power consumption by interposing between one face of the unit andone of the electrodes, preferably the upstream electrode, anotherdielectric body.

The separator, therefore, also includes a dielectric body, preferably inthe form of a series of laterally spaced parallel glass plates 1,extending in the direction of the gas flow. While I prefer the use ofglass as a dielectric, it is to be understood that various otherdielectric materials, including wood, have been and may be successfullyused. The dielectric body 1 should have its opposed faces in contactwith, or minutely spaced from both the upstream electrode and theupstream face of the filtering unit. The plates should be placed asclose together as they can be without interposing any appreciableobstruction to the air flow. Ordinarily, a lateral spacing substantiallyunder 2" and preferably around 1A" will be satisfactory.

The faces of the glass plates 1 may be coated if desired with relativelysmall particles of a different dielectric material such, for example, ascrude vermiculite i. e. a flaky micaceous mineral. A `coating ofparticulate or discrete dielectric material while not essential is,therefore, shown as indicated. at 8 in Figure 2.

The dielectric separator, as a whole, should occupy substantially theentire gas flow depth of the electric field between electrodes, andshould either provide good contact throughout that depth oryavoid suchcontact by minute clearances only. In the arrangement shown,satisfactory contact between the upstream face of the separator'and theupstream electrode is easily secured. Due to surface irregularities ofthe filter unit, satisfactory contact or clearance between each face ofthe filter unit and the opposed faees of adjacent parts may not be soreadily obtained. To insure good contact, a somewhat resilient,interstitial body 9 can be interposed and slightly compressed betweeneach face of the lter unit and the adjacent faces of the dielectric 1 onone side and the downstream electrode 2 on the other. The interposedbody 9 in each case may be made of material having either electricallyconductive or dielectric properties. Where electrically conductivematerials are used, they are preferably made in the form of a resilientpad or sheet, such, for example, as a pad or sheet of steel wool or ofcopper mesh fabric, the latter material being of the character shown inPatent No. 1,676,191 granted July 3, 1928.

The gas iiow depth of the foregoing arrange.

ment may be varied widely within safe limits without appearingappreciably to affect the effectiveness of its operation. For example,with an operating voltage of 12,000 volts, the cleaning results obtainedwhen the air flow depth of the glass plates was 11/2", o'f the iilterunit 4" and of the'device, as a whole, 6", were not noticeably variedeven after the overall air flow depth was increased to 10" bycorrespondingly increasing the depth of the plates. No attempt is hereinmade to explain principle governing the operation of this device. It hasbeen noted, however, that in operation particles are effectivelydeposited on the filter medium, principally in the areas where it eithercontacts other surfaces or is minutely spaced therefrom. Particledeposits have been noted not only at the points of contact or minutespacing between the filter medium and the adjacent upstream anddownstream surfaces, but also at all points of contact or minute spacingbetween the filter medium l and other material such as the spacers 5.

In the'operation of this gas cleaner, it has also been observed: thatozoneis not produced in noticeable amounts if at all; that currentvariations, ranging from one microampere to one-half milliampere, appearto produce no observable change in its effectiveness; and that theeffectiveness of the device is not instantly gained or lost uponconnection to or disconnection from Y power, but on the contrary isgradually built up or dissipated over a period of time on the order often minutes more or' less. Consequently, although the underlyingprinciple of operation has not been definitely ascertained, theseobservations may indicate that ionization is not the determining factor.It has been suggested that the effect may be due to polar or chargedparticles being forced from the air stream in a non-uniform electriceld. It may be, and I am strongly inclined to believe that it is, due tothe distribution of charges of static electricity over the surface ofthe lter medium. From all indications, it would appear that charges ofstatic electricity are distributed over the medium, that dustparticlesfare effectively precipitated when brought under the influenceof such charges, and

that the emciency of 'separation is proportional to what I term theelectrostatic potential of the medium, thatis to say the number ofcharges which can be maintained upon the medium. All

this is indicated by the results secured in opy erating the arrangementsshown in Figures 6 and 7.

I have discovered that the arrangement shown in Figures 1- through 5 maybegreatly improved by eliminating the material `interposed between thedielectric kfilter medium 'I and the charging' electrodes I` and 2, andby placing'such electrodes into direct .contact with the filter mev diumor otherwise causing themediumto be charged with an effectiveelectrostatic potentialff An Aarrangement of` this character isschematically indicated in Figure 6.

In this arrangement,` good results have been obtainedwith filter mediasuch as Airmat," the paper disclosed in Patent #1,897,976, felted wool,

canton flannel, organdy, voile, rayon, matted` 5 composed wholly ofshort fibers or having a mul- 1 titude of such .fibers forming a napover its surface. While upstream and downstream electrodes in the formof window-screen may be employed, electrodes composed of laterallyspaced rods or wires are preferred' and illustrated in Figure 6. Theseelectrodes maybe disposed on the same side of the medium, on oppositesides thereof, or on both sides, but, in any event, they preferably areplaced in good contact with the medium along their length and laterallyspaced sufficiently to prevent fiash-over. A direct 4current chargingvoltage of 12,000 volts is preferred, although this may be varied up ordown as desired.

I have also discovered that a` conventional ionizer may be used in placeof,or in combination with the charging electrodes I and 2 with goodeffect provided it is placed sufiiciently close to the filter medium tocharge it with an eective electrostatic potential. In this connection,it

may be noted that electrical air cleaners, having ionizers arranged inadvance of cloth and other like filter mediums, have heretofore beenproposed. In these cleaners, however, the ionizer 3o operates toagglomerate fume and smoke particles to a size which'facilitates theirmechanical flltration from the air. So 'far as is known, thesearrangements contemplate the use of low air- K fiow velocities and theprovision of substantial spacing between the ionizer and filter mediumin A order to allow sulcient time for the agglomeration to take place.Even then, the size of the agglomerates is so small as to require adense filter medium to effect their mechanical filtra- 40 tion from theair.

\ I have discovered that when a dielectric filter medium i's placedclose to the downstream side of an ionizer, a substantial number of theelectrostatic charges generated by the ionizer collect upon the ltermedium, thus charging it with an electrostatic potential whichiseffective to `precipitate substantialquantities of smoke andfume,`whereas, when it is spaced a substantial distance away from thedownstream side of the ionizer, most, if not all, of the generatedFcharges are dissipated in some way before they 4reach the filtermedium, with the result that it is charged with little or noelectrostatic potential and, therefore, ineffective to precipitate evensmall quantitiesof smoke and fume. Ac-

v cordingly, when its use is desired, a conventional ionizer, havinglargeand small electrodes I0 and AI I, is arranged close enough to theupstream face of the filter medium to charge it with an effective 60.-electrostatic potential.

` A practicalembodiment of the structure schematically illustrated inFigure' is shown in Fig- Q ures '7 and '8. In this embodiment, an airAfilter of the 'type shown in the Nutting Patent 6;, .#2,211,382 grantedAugust 13, 1940, is used, this filter comprising: aunitary'open-endedvair inlet frame M having outer side walls forconfining -the airpassing through the frame, two laterally spaced sidewalls `being serrated to form two 70,1laterally spaced series of fingersvI2 extending in the direction of air flow: another unitary open-endedair outlet frame W having corresponding side walls, serrations andfingers I3, the latter being matable with the Ifingers I2 of the frame Mand cooperating therewith, when the frames M`and W are mated, to pinchthe intervening portion of a filter medium I interposed between theframes; and a corrugated screen I4 on the frame W to support'the filtermedium against the force ofthe air flow.

Good results have been obtained with Aa dielectric filter medium 4interposed between all metal frames M and W when such frames wereoperated in connection with an ionizer and either connected to ground ordisconnected therefrom. For purposes of safety, however, a groundconnection to the all metal frame is preferred.

In Figures 7 and 8, however, the filter illustrated is not an all metalfilter but, on the contrary, is an insulated filter in which the ngersI2 and I3 are composed of some insulating material such as Bakelitewhile a Bakelite screen? is placed on the air-outlet frame W in place ofthe metal screen normally used. Additionally, the frames M and W arerespectively provided with charging electrodes I and 2. With thisarrangement, good results have been obtained when either set or bothsets of electrodes were energized with direct current at 12,000 volts.For example, with a filter 24 x 24" in cross section, fitted withinawooden duct'D and positioned with its upstream face spaced 4" from thedownstream face of the ionizer (or 9" from a vertical plane passingthrough the longitudinal axes of the ionizing electrodes I and II) andwith both sets of electrodes energized by a direct current of 12,000volts, cleaning-eidciency tests were conducted with various dielectricfilter mediums using atmospheric air as the medium to be cleaned. Thecleaning efficiencies obtained were measured by the discoloration methodwhich is published in the August 1938 edition of the Joumal Section ofthe Heating, Piping and Air conditioning magazine. These results follow:

Per cent cleaning efficiencies Filter medium Sub' Initial quent Wovenglass Below 40 Y 40 Ms" ply matted glass. 43 44 Rayon. 50 50 Voile l 60Org dy 60 60 14 oz Ie ted wool 55 65 Same with surface roughened..- 8082. 5 Same with surface rcsmoothed- 75 75 l2 oz. canton flannel 65 90Same with surface singed 65 80 10 ply Airma 85 85 It will be understoodthat the emciencies shown by the discoloration method indicate theability of the filter medium to remove smoke, fume and other extremelyfine particles which cannot be effectively removed from air bymechanical illtration, When-the same test method is employed to measurethe cleaning efficiencies of any-of the mediums above noted, such asAirmat, under conditions in which it is not subjected to anelectrostatic charge or potential,

,the efficiencies obtained are substantially unaceaeci to ground,whereas this current dropped to 0.1 micro-amperes when such spacing wasincreased to 9". Again with an all metal filter connected to ground andhaving its upstream face spaced 3", 6" and 9" from a vertical planepassing' through th'e longitudinal axes of ionizing electrodes I0 and IIenergized at 20,000 volts, leakage currents of 100, 10 and 5micro-amperes respectively were observed'in the ground connection of thefilter. The amount of current in such ground 4connections appears to bean indication of the electrostatic potential of the filter medium,

and, as such, shows that such potential decreases rapidly with increasesin the spacing between the filter and the ionizer. Nevertheless, whilethis is true, a small leakage current should not be taken as an absoluteindication that the electrostatic potential of the filter mediumisineffective, because the magnitude of the leakage current not onlydepends upon the magnitude of the electrostatic potential but upon theresistance afforded tothe flow of current. In this connection, it hasbeen noted that the amount of leakage current normally'increases with anincrease in the humidity of the atmosphere, although, at the same time,the efliciencies of the filter medium, and, no doubt, the magnitude ofthe electrostatic potential, decreases.

Generally speaking, it may be said that good results will always beobtained when an effective electrostatic potential is maintained on thefilter medium, and that the maintenance of an effective electrostaticpotential will be promoted by using short-fibered or napped filtermediums, by using the highest practical voltage or by increasing thedryness of the air o r gas being cleaned, where this is possible, and bydecreasing the spacing between the filter medium and the ionizer whenthe latter is used. Since the maximum permissiblespacing between theionizer and filter medium will depend upon various factors which aifectthe dissipation of the generated charge and since such factors will varyin different installations, it is diiiicult accurately to define the 0maximum spacing limit; hence, in each particular installation, it may benecessary to determine such limit experimentally should conditions re.-quire the use of maximum spacing. In this connection, it may be notedthat the generated charge apparently was entirely dissipated when aspacing of 15 feet was employed between an ionizer and filter medium ina metal duct having an area 24" x 24". It should suilice to say that thesmaller spacings are recommended, that a minimum of metal be interposedbetween the ionizer and `filter medium in order to minimize thedissipation of the charge and that, to promote safety, all interposedmetal be grounded.

Having described my invention, I claim:

1. An electric gas cleaner comprising: spaced upstream and downstreaminterstitial electrodes; an interstitial dielectric body arranged withinthe electric held space between electrodes and substantially in contactwith the upstream electrode; and an interstitial dielectric air filtermedium interposed between and in electrical contact with said dielectricbody and said downstream electrode.

2. An electric gas cleaner comprising: spaced upstream and downstreaminterstitial electrodes; a series of laterally spaced substantiallyparallel dielectric plates extending from the upstream electrode towardthe downstream electrode; and

'a dielectric air filter medium arranged between aser/,eci

and in contact with the downstream electrode and the downstream edges ofsaid plates.

3. A device as claimed i'n claim 2, wherein said lmedium is composed offibrous material;

4. A device as claimed in claim 2, wherein said medium is composed ofshort bred material matted to form a. highly porous net work.

5. The combination with' a gas ow duct of an electrical gas cleaningdevice comprising: spaced upstream and downstream interstitialelectrodes, each electrode extending laterally across the gas ow spaceof said duct; and an interstitial dielectric separator substantiallyoccupying -the gas flow space of the duct between electrodes.

6. The combination with `a gas flow duct of an electrical gas cleaningdevice comprising: an interstitial dielectric lter medium with upstreamandgdownstream air-flow faces extending across and means to establishand maintain an electrostatic potential on said idter medium tosubstantially increase the collecting ability of said filter medium forrelatively iine particles including particles of smaller size than theinterstices in said iilter medi said duct to lter the gas passingtherethrough, l

7. The combination dened in claim 6 wherein: -said means includecharging electrodes in physical contact with said dielectric illtermedium.

8. 'I'he combination deiined in claim 6 wherein: said meansincludes anionizer ir. said gasiiow duct positioned upstream of said dielectricillter medium and suiiiciently close to the upstream face of. saiddielectric lter medium to prevent the dissipation of charges imposed onthe particles in the gas ow by said ionizer prior to their reaching saiddielectric iilter medium.

CHARLES W. WILLIAMS

